Aerial Archaeology in Ireland - The Heritage Council

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Metrical Survey
As Doody observed, the simple question of ‘yield’ in terms of the number of sites identified is only part of the issue. Compared
with identifying sites, there is a big difference in the relative value of oblique and vertical photographs for accurately locating
and surveying them, as we have seen (Figures 26, 27). While oblique photography is better at exploiting lighting effects to
maximise clarity (Wilson 2000), vertical photography offers other advantages. It is more systematic, and offers enhanced
stereoscopic viewing and greater ease in rectifying and plotting results (including, for example, digitally integrating scanned
photographic images with terrain modelling). It may also be noted that very high-resolution colour and panchromatic satellite
imagery such as QuickBird can now produce imagery in the visible spectrum to 0.6m resolution, which is better than standard
OSi ortho-photographic mapping (Section 3.6).
Much depends on what levels of accuracy are needed. Oblique photographs are often better for identification in terms of
visibility of features. They can also be rectified to provide approximate positions on medium to large-scale maps, if that is all
that is needed. But their use as a means of accurately surveying and measuring a monument to a high degree of accuracy (e.g.
to within a few metres) is often suspect, especially on non-flat ground. For example, rectifying oblique aerial images to 1:10,000
or larger scale maps will result in inaccurate locations for sites due to the errors in the base map itself, the change in scale
throughout the image, and the errors due to relief displacement. Not all of this can be allowed for, even in bespoke rectification
software, without the additional information provided by digital terrain models.
In the past few years, the Discovery Programme has undertaken various aerial survey projects — notably at North Roscommon
and Mullaghfarna, Co. Sligo — which show the considerable potential of large-format vertical photography in mapping
archaeology (Figures 13, 32). Each of these projects was carried out using professional photogrammetric software. Medium
and/or low-level photography were commissioned, with ground control provided by DGPS. The resulting mapping has produced
true ortho-photos to a very high level of accuracy (each pixel covering 0.16m x 0.16m on the ground) that is better than base
mapping tolerance. The new mapping has revealed a level of archaeological landscape and monument detail that was well
beyond expectations. In the North Roscommon Project, it was found that oblique images could only be rectified to a good level
of accuracy when additional control points, identified from ortho-photography derived from vertical images, were used.
3.5 Lidar
Principles and Strengths of Lidar
This technique generates high-resolution elevation data derived from a special airborne instrument that samples the underlying
ground height using the return time of a laser pulse across a swathe of land (Figure 29). The data are automatically georeferenced
by means of an on-board receiver linked to the Global Positioning System (GPS) provided by earth-orbiting satellites.
Sample points are typically interpolated at 2m horizontal intervals, with a vertical error range of only 10-15cm which compares
very favourably with conventional vertical photography. This provides a reasonably accurate means of gathering 3-dimensional
co-ordinates of the ground surface to produce Digital Terrain Models (DTMs), though the resolution is not as good as detailed
ground-level surveying. Lidar data often contain a degree of background noise. Vegetation, buildings and other objects may
need to be removed to detect the true land surface, and this can be achieved by filtering and interpolation techniques relevant
to the actual application.
For each point recorded, the lidar data provides the horizontal position of each measurement point at horizontal intervals of
c. 0.5m on the ground, and first return and last returns of the laser pulse and their intensity. This allows both solid (ground)
profiles to be distinguished from vegetation and measured to within 15cm absolute accuracy. The intensity measurements help
to establish different types of vegetation or structure.